docs: fix typo in Redis configuration

João Pereirarequested to merge
redis-docs into master

For some reason, we had the "main name" Redis configuration parameter key documented as mainName, instead of mainname. We always look for mainname in the code (https://gitlab.com/gitlab-org/container-registry/-/blob/b120065dffe3c9a63e43050bb518b9857327ad59/configuration/configuration.go#L308). This MR fixes the documentation.

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